Nuclear fuel elements generally include small kernels of fissile or fertile materials coated with one or more layers of carbon and/or ceramics to form fuel particles that are held in a graphite matrix. FIG. 1 illustrates a typical fuel particle 10 that includes an inner kernel 12 of fissile or fertile material. A typical fissile material is a mixture of uranium oxide or uranium carbide, and a fertile material is generally thorium oxide, which is converted to fissile uranium upon irradiation with neutrons. The inner kernel 12 is usually quite small, on the order of about 500 micrometers in diameter. The inner kernel 12 is surrounded by layers of carbon and silicon carbide that serve to contain the radioactive fission products formed during use. Recent developments are considering replacing the silicon carbide with zirconium carbide. For example, as shown in FIG. 1, the inner kernel 12 is surrounded by a first layer 14 of a porous carbon buffer and a layer 18 of pyrolytic carbon. The fuel particle also includes a layer 16 of a ceramic material, generally silicon carbide, and an outer layer 20 of pyrolitic carbon. The outer layers of the particles are generally on the order of about 30-50 micrometers thick each and an individual particle will be on the order of about 900 to 1000 micrometers in diameter.
Fuel particles as illustrated in FIG. 1 are embedded in a graphite matrix. In one common configuration, the particles are dispersed into a graphite matrix and formed into compacts that are generally about 10 to 50 millimeters in diameter and about 50 to 100 millimeters high. The compacts are then embedded into graphite blocks to form a fuel element. Fuel elements of different shapes and sizes have also been developed. A second common configuration is the pebble bed reactor, which utilize fuel elements in the form of spheres in which the fuel particles are embedded in a spherical graphite matrix of about tennis ball or billiard ball size (e.g., about 60 millimeters in diameter).
Whatever the final form of the fuel element, it is necessary to process the used fuel elements to reduce the volume of the radioactive waste for disposal (e.g., the fuel particle content of the fuel element is typically less than 5% by volume) as well as to render the radioactive materials into a safe form for long-term storage. Fuel element processing includes separation of the kernel material from at least the graphite matrix surrounding the fuel particles and, ideally, also from the silicon carbide and carbon layers for recovery and/or disposal.
Methods utilized in the past have included mechanical crushing of the matrix and particle outer layer materials followed by high temperature incineration or gasification of the carbonaceous materials. Unfortunately, methods utilized to date present difficulties. For instance, mechanical crushing of the fuel element can produce fines that require expensive containment procedures and the high temperature processing methods can be quite expensive. Moreover, once the silicon carbide containment layer has been breached, any materials still in contact with the particles must be treated as high level waste.
What is needed in the art is a method for processing used graphite-based fuel elements. For instance, a lower temperature treatment method that can efficiently remove carbon and ceramic encapsulation materials from a fuel kernel would be of great benefit. Moreover, a method that can remove low level waste materials from the fuel particle prior to breach of the silicon carbide layer, so as to prevent contamination of the low level waste with the interior high level waste, would be of use.